This mod removed the limitation of commands to modify player NBT data.

For example, this mod allows you to use this command:
/data modify entity @s Inventory [{id:"minecraft:stone",count:64,Slot:0b}]
Which gives you an stack of stone in the first slot of your hotbar.

You can use this for exampel to save your current inventory:
/data modify storage inventories Player1 set from entity YourPlayerName Inventory

And load back your saved inventory at any time using:
/data modify entity YourPlayerName Inventory set from storage inventories Player1

Important

Some fields of the players NBT data are strictly read only even with this mod, simply because that is how the game works.

These are mainly fields that represent properties which are determined by additonal factors.

mayFly, instabuild and mayBuild are for example determined by the game mode of the player, and can not be edited, only read.

Postion, Motion and Rotation also can not be edited.

But most other fields work like on any other entitiy, this includes, but is not limited to:
Inventory
Health
foodLevel
Score
XpLevel
Air
EnderItems
And many more.

PlayerNBT